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Subject: Re: [PATCH v13 09/12] mm: hugetlb: add a kernel parameter
 hugetlb_free_vmemmap
To:     Muchun Song <songmuchun@bytedance.com>, corbet@lwn.net,
        mike.kravetz@oracle.com, tglx@linutronix.de, mingo@redhat.com,
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        paulmck@kernel.org, mchehab+huawei@kernel.org,
        pawan.kumar.gupta@linux.intel.com, rdunlap@infradead.org,
        oneukum@suse.com, anshuman.khandual@arm.com, jroedel@suse.de,
        almasrymina@google.com, rientjes@google.com, willy@infradead.org,
        osalvador@suse.de, mhocko@suse.com, song.bao.hua@hisilicon.com,
        naoya.horiguchi@nec.com
Cc:     duanxiongchun@bytedance.com, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        linux-fsdevel@vger.kernel.org
References: <20210117151053.24600-1-songmuchun@bytedance.com>
 <20210117151053.24600-10-songmuchun@bytedance.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Message-ID: <7550ebba-fdb5-0dc9-a517-dda56bd105d9@redhat.com>
Date:   Mon, 25 Jan 2021 12:43:23 +0100
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On 17.01.21 16:10, Muchun Song wrote:
> Add a kernel parameter hugetlb_free_vmemmap to enable the feature of
> freeing unused vmemmap pages associated with each hugetlb page on boot.

The description completely lacks a description of the changes performed
in arch/x86/mm/init_64.c.

[...]

> --- a/arch/x86/mm/init_64.c
> +++ b/arch/x86/mm/init_64.c
> @@ -34,6 +34,7 @@
>  #include <linux/gfp.h>
>  #include <linux/kcore.h>
>  #include <linux/bootmem_info.h>
> +#include <linux/hugetlb.h>
>  
>  #include <asm/processor.h>
>  #include <asm/bios_ebda.h>
> @@ -1557,7 +1558,8 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
>  {
>  	int err;
>  
> -	if (end - start < PAGES_PER_SECTION * sizeof(struct page))
> +	if (is_hugetlb_free_vmemmap_enabled() ||
> +	    end - start < PAGES_PER_SECTION * sizeof(struct page))

This looks irresponsible. You ignore any altmap, even though current
altmap users (ZONE_DEVICE) will not actually result in applicable
vmemmaps that huge pages could ever use.

Why do you ignore the altmap completely? This has to be properly
documented, but IMHO it's not even the right approach to mess with
altmap here.

-- 
Thanks,

David / dhildenb